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Making a microscope without a lens

Researchers make a microscope that beats conventional microscopes, but it has …

Those of you who have suffered through my writing before will know that I have a thing for imaging. A paper that offered experimental results on a new super-resolution technique was like offering me free beer: irresistible. Even better, it turns out that we have coveredalmost every major bit of research that has led to this new work.

What do I mean by superresolution? Well, a lens has a focal length and a diameter. To create a high resolution image, you want the largest possible diameter combined with the shortest possible focal length. The result is a lens that collects light from a large cone of angles, allowing us to construct an image with more closely spaced features. Unfortunately, there is an upper limit to the combinations of lens diameter and focal length that limits resolution to about half the wavelength of the light used to image—so, for a visible light microscope with the best lenses and a few other tricks, the best resolution is about 200nm. This limit is called the diffraction limit. Superresolution refers to techniques that allow one to beat this limit.

This latest bit of work from the University of Twente is very different though, because it doesn't really use a lens, yet it achieves a resolution that is about twice what you would expect under optimum conditions. How was this achieved? Through the power of scattering and the magic of wavefront shaping.

Focusing through scattering

Light scattering is usually something to be avoided. Every time light hits an interface between two materials, a portion of the light is reflected and a portion of the light is transmitted at some angle. Once it encounters enough interfaces, the light ends up going in every direction. Take two photons entering at nearly the same location with the same angle of entry, and they will exit at two completely random locations heading in two random directions. This makes these materials appear white—like sugar cubes and flour.

When you do this with a laser, you get a speckle pattern because all the different paths through the material mean that, when the light overlaps with itself outside the material, some parts are in phase with each other (creating bright spots) and others are out of phase (creating dark spots), but most are somewhere in between (creating a dull glow). This is where the power of wavefront shaping comes into its own.

Essentially—if you want more details, you should click some of the links in the first paragraph—if you can figure out how long all the different scattering paths through the crystal are, you can correct for the scattering. To do this, you pass the laser beam through a liquid crystal display. Each pixel can modify the local phase of the light—this is wavefront shaping. By doing this, you can create the opposite of the scattering that the light is about to undergo in the sample. The scattering material then undoes all the adjustments that the liquid crystal made, and the beam appears to have passed through both unchanged—well, except for the fact that most of the light disappears in the process, but that is a small detail, right?

But if you can do that, you can do anything. Want your scattering material to be a lens? No problem: just set your wavefront shapes appropriately, pass the beam through the scattering material, and a focus is yours to be had. The downside is that the only way to figure out the right settings for the liquid crystal screen is to put a camera at the focus and adjust until a bright spot is observed. This leads to a chicken-and-egg scenario: you need to see that you have a focus to get a focus, but you need a focus to see.

Ignoring that problem for the moment, the important thing is that, because the scattering light can exit the material at a very wide range of angles, one can create a focus that is much closer to the scattering material than possible with the equivalent diameter lens. The result is that the focus is constructed from a much larger cone of angles, allowing one to resolve smaller features.

The hardware

Admittedly, you need to use materials that have a huge refractive index, so they aren't that convenient to work with. In this case, the researchers used a Gallium Phosphide (GaP) wafer, one side of which had been dipped in acid to create a scattering layer. A thin layer of silicon was deposited on the other side, because, although the wavefront shaping creates a focus, not much of the light ends up in the focus. The rest of the light bounces around until it is detected, swamping any potential image. Silicon absorbs almost all of the light incident on the flat side of the wafer. A small area of silicon was etched away to create a non-absorbing area where a sample of gold spheres was placed.

This is where the downsides start to appear: the field of view was really small—in the paper, the researchers only image over two micrometers squared. In a normal microscope, an image is often created by raster scanning a laser over the sample. But here, the scanning range is limited, because as you move the laser beam, the scattering paths vary.

You might think that this destroys the focus, but over a limited range, the focus remains as sharp as allowed by the scattering materials. However, as the laser is moved, less of the light ends up in the focus and it instead contributes to an omnipresent background that must be overcome. In the end, you have no light in the focus anymore and can't image, so to image wider fields, one would have to scan the entire setup.

The other thing to note is that the researchers used gold as their sample. Why? Because gold nanoparticles glow like a flare when light hits them, making it easy to figure out the settings required for the wavefront shaper to provide a good focus—just maximize the big bright spot created by the glowing gold spheres.

So, how well did they do? Well, better than a conventional microscope, as you might expect, since I am writing about their work. But, only by a factor of just over two (~100nm), which, is, in some ways, quite disappointing. There is a technique called structured illumination microscopy that essentially does the same thing, but it is much simpler and has fewer of the disadvantages. Structured illumination microscopy gives you exactly a factor of two over the standard diffraction-limit.

The other issue that is not mentioned is depth of field. In this example, the sample was right on the lens surface. This is necessary, because you need the high refractive index material to get the focus. So how do you use this to image inside a cell, for instance? At the moment, you couldn't.

If I seem to be ending this negatively, I don't want to give the wrong impression. First, the technique does do a little better that structured illumination microscopy. Second, there are no expensive lenses in this experiment. What this really proves is that if you have a good scattering wafer, a wavefront shaper can compensate for low quality optics. Since wavefront shapers can be made from MEMS mirror sets that are used in commercial beamers, we may well be looking at the beginning of a new age of very cheap, high resolving power microscopy.

ArXiv.org (formatted for Physical Review Letters at the time of writing), 2011, 1103.3643

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He Lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku